It is known that in the orthodontic treatments, such pieces like brackets, buccal tubes or apparatuses of this type must be adhered to the teeth by cementing. The specific parts to be joined are in the anterior or posterior surface of the tooth and one surface of the bracket which is known as the dental surface of the base. The base is intimately joined to the so called bracket body wherein the retention groove of the orthodontic arc is generally to be found.
Taking into consideration the manufacturing materials, the brackets can be metallic, ceramic or plastic, but due to economic and aesthetic reasons the trend is more and more to produce them with plastics since the material is cheaper than metal or ceramics and also can achieve colours similar to the one of the teeth or yet better make them transparent, which gives them an added aesthetic value. A technological challenge of the producers of orthodontic devices is therefore to achieve a plastic bracket that provides for characteristics similar to the metallic ones with respect to deformation resistance and/or breaking strength and retention properties, but moreover being more aesthetic and showing less manufacturing cost. It is also important to take into consideration the manufacturing procedure, which can be by means of mechanising or injection moulding, the latter being less expensive.
The queries to be considered when it comes to achieve a plastic bracket with the aforementioned properties are fundamentally: the shape of the base, the shape of the bracket body, the type of plastic in the moulding process for its manufacturing.
It is a primary object of the present invention to achieve that the bracket has a base of mechanical retention. If one analyzes the base, a first important aspect to consider is the adherence power thereof with the tooth, said base having to support forces within the range of 50-100 newtons. This is not easy to achieve, taking into consideration the material and the small size of the piece. Because of that the form of the base and particularly that of the dental surface have been the object of several patents, since the retention strength as well as the easiness of implantation depend on said form. The retention may be chemical or mechanical and is a factor that the professional takes into consideration when he chooses a determined type of bracket; since the chemical retention comprises the additional step of applying a liquid to the base for the joining of the base with the cementing material can be produced.
The body of the bracket has generally some retention wings and a groove for the arc that conform to a characteristic geometric figure in which the zones with a greater deformation or breaking probability are delimited when the bracket is subjected to the tensions of the arch that is introduced into the groove and to the chewing. It is known that the plastic is easier to be deformed and broken than metal or ceramics, but there exists moreover the additional problem that a plastic bracket may suffer a progressive degradation within the time due to hydrolysis, that in the same time causes mechanical defects and a progressive change of the colour of the plastic up to turning opaque. The degradation by hydrolysis is avoided by means of a finishing of the part that is as perfect as possible, and here comes to play a role the material selected as well as the moulding process selected.
It has been said already that the form of the base depends on the union of the bracket with the tooth, and that this form has been the object of different patents. The U.S. Pat. No. 4,544,353 discloses an apparatus that improves the technique of joining by direct contact the teeth with an orthodontic device. The joining technique by direct contact consists in applying the adhesive onto the dental surface of the base; putting the base covered with the adhesive onto the tooth; and cleans the residual adhesive. Due to the form of the base of the orthodontic devices, which were existing up to that moment, this operation became somewhat difficult because of the sliding of the glue and the bracket during the positioning thereof on the tooth's surface, and moreover having to clean the remaining adhesive. In the mentioned patent, this problem is resolved by means of a base, which has a flange with an angle between 90° to 70° which extends along its whole periphery, thereby defining a cavity that receives the adhesive and that in addition to cutting off any exceed of adhesive it impedes its falling during the positioning of the tooth.
The U.S. Pat. No. 5,295,824 discloses a plastic bracket that improves the joining process of a plastic bracket with the teeth, in which the union surface of the tooth is covered with a mixture of acrylic monomers and solvents that attack and dissolve the plastic substrate, thereby being spread within the same. Once the dental surface of the base is covered, the bracket and the mixture are heated in order to volatilize the solvents and monomers of lower molecular weight thereby achieving a layer of acrylic material that is predominantly monomeric. This first layer acts as a first adhesive and improves the joining strength when the same is performed with dental adhesives such as the acrylic ones. The appropriate solvents cited in said patent are benzene, chloroform, acetone, dichloromethane or any other acting as solvent of plastics composing the bracket. The plastic brackets made of polycarbonate are very appropriate for using this method.
An objective being searched for through the years is to achieve plastic brackets whose base has a dental surface sufficiently rough in order to not needing to vulcanize the surface before fixing it to the tooth. The process is tried to be analogous to the one of the metallic brackets which in general have some grooves in the dental surface of the base that are sufficient to obtain a cementing that provides for the adequate retention strength.
The U.S. Pat. No. 6,071,117 proposes a method for manufacturing orthodontic devices made of plastic by means of injection moulding; the surface of the base of the bracket projects itself outwards with such forms like cylinders or rectangular prisms that are easily configurable since they are created in a mould. This form of surface of the base expects the joining surface of the bracket's base with the fixing cement to be the largest possible. A first problem is caused because of the fact that the surfaces of the aforementioned forms cannot be rough since if the mould would be designed in this way, some breakage could be caused while taking the bracket out of the mould. A consequence of this is that the achieved geometry by means of the moulding is not sufficient to obtain the adequate strengths if the retention is mechanical. To resolve this problem, a second step is carried out, which is to deform the geometry of the base in the external part by means of heat or ultrasonic energy and to provide for spaces with the shape of an inverted cone next to the external surface, that facilitate the mechanic retention of the base of the bracket with the surface of the tooth.
Possibly because of problems with a mechanical retention, the U.S. Pat. No. 6,109,165 of the same applicant of the aforementioned patent discloses a similar method with respect to the form of the base and the deformation process, that includes one step more, which is posterior to the foregoing, and consists in bombarding with particles in order to make them crash against the external part of the base's projections.
The method disclosed in said foregoing U.S. Pat. No. 6,071,117 and U.S. Pat. No. 6,190,165 has the problem of to the step of moulding there is the need of causing the deformation, with what, although the deformation is carried out in deformation stations that control the process, the heating element may reach up to 400° F. (204° C.), which might cause deformations in the whole bracket.
A second object of the present invention is to obtain mechanical facilities which are equivalent to those of the metallic bracket. If we consider the problem that is posed by the hardness of the plastic bracket, or what is the same, the ability of resistance to deformations or breakage, this one has been fundamentally treated in two ways. One of them is to look for the appropriate type of plastic and the other one is to introduce into its interior metallic structures that reinforce the points that have to support higher tensions.
When the type of plastic is mentioned, it is not only talked about whether it is from the class of polycarbonates, polysulfones, polyethersulfones or others, but what is meant is that the plastic may or may not be charged with other elements. The charge doesn't involve the mixing with plastic of inorganic materials like the alum or silica, although it may also be charged with organic or metallic materials, always with the purpose of augmenting the resistance of the assembly.
The U.S. Pat. No. 5,254,002 as well as some embodiments of the aforementioned U.S. Pat. No. 6,071,017 and U.S. Pat. No. 6,190,165 disclose a method for reinforcing the plastic in which the plastic is a refilled by dispersing in its interior some inorganic materials such like glass particles. Nevertheless, this method of dispersion of particles inside the plastic be it from glass, alum or metallic ones, has the problem that micro-breakages are caused due to the fact that, when being injected, the mixture has a high temperature and as every component has a different coefficient of expansion, some deformations are caused as it cools down. This effect is more intense in the narrower zones of the geometry of the body which on the other hands are those being more likely to break, and in the superficial zones as well, which causes an increment of the non desired effect of hydrolysis.
In order to alleviate the aforementioned effect of micro-breakages the U.S. Pat. No. 6,358,043 introduces into the mixture organic materials by mixing polycarbonate as basic element and an elastomer from styrene as a hardening element of a particle size of 0.1 μm to 10 μm. Nevertheless, the hardening does not seem to be sufficient, as it includes a reinforcing metallic sheet which extends inserted between the wings in the direction of the groove. One can list a series of problems of the brackets that have elements embedded with metallic reinforcement when fabricated by means of injection moulding. Among said problems one can list some like deformations in the plastic matrix, chips, movements of said sheet during the manufacturing process which even can cause the movement of said sheet during its use in an orthodontic treatment process, and also the raising of its price.